Telephone subscriber stations in general include components for signalling an incoming call, for indicating the busy or idle condition of the station, for directing destinations, and for communication with other subscriber stations. Among the components used to facilitate the communication between connected subscriber stations are inductive elements, such as transformers. These inductive elements, besides being costly, are also cumbersome, especially in comparison with modern solid state and integrated circuitry. Because inductive components are not amenable to hybridization, their inclusion in a circuit limits the otherwise obtainable advantages of solid state miniaturization and hybridization.
The inductive elements are used for matching the load of subscriber station networks. The load includes the telephone lines coupling the subscriber stations to the central office. The characteristics of the lines can vary depending upon such variables as ambient climatic conditions. Networks using inductive components are used to match the substation to an average line length and ambient condition. The inductive components are not readily variable, because of cost and physical condition. Thus, the average substation is typically not properly matched to the telephone line.
The condition of the substation that is not properly matched which causes the most difficulties occurs where the transmission and reception of communication signals is overly attenuated. With the advent of solid state electronics, it is easier to equip the substation with amplifiers to aid in overcoming the impedance matching problems. This, of course, is an expensive way of doing things, so that the cost effectiveness of the system rapidly deteriorates.
Inductors are also traditionally used to provide the 4-line to 2-line conversion necessary for coupling the transmitter and the receiver of the subset to the bi-directional 2-line phone lines linking the subset to the central office. Such elements are required to provide isolation between the simultaneously transmitted and received signals.
Where these coupling inductors have been eliminated, complex gyrator or bridge circuit configurations utilizing typically several solid state active devices have been used. One such circuit based on this concept is disclosed in U.S. Pat. No. 3,823,272 entitled "Electronic Telephone Transmission Circuit" invented by C. M. Tabalba issued July 9, 1974. Another is disclosed in U.S. Pat. No. 4,071,713 entitled "Telephone Speech Network" invented by M. G. Sencer issued Jan. 31, 1978. Such prior art circuits are not amenable to implementation in simple series and series/parallel configurations utilizing as few as two transistors.
Accordingly, an object of the present invention is to provide new and unique electronic subscriber station networks.
Yet another object of the present invention is to provide solid state networks for subscriber stations with the transmit and receive signals almost constant independent of variations in the line length.
Still another object of the present invention is to provide amplification both for the transmitter and receiver in the subscriber subset.
Still another object of the present invention is to provide electronic network circuits which limit the current going through the transmitter, thereby preventing its overload. The current limiting, which is inherently provided by the electronic circuit network used for controlling load matching and sidetone, also eliminates the need for transmitter overload protection.
Another object of the present invention is to effectuate all necessary transmitting and receiving functions of the subscriber subset by simple series and parallel circuit configurations, eliminating the bridge circuit configurations typically employed.
Another object of the present invention is to minimize the circuitry required to perform the objects of this invention, thereby increasing its cost effectiveness in mass production.
Yet another related object of the present invention is to achieve circuit minimization by utilizing the transmitting circuitry as well as the receiving circuitry to control the level of the signal through the receiver.
Yet another related object of the present invention is to use the same circuitry, without additional active devices, to control the amplitude of the sidetone and make it almost constant irrespective of line impedance, by utilizing in-phase control signals.
Still another object of the present invention is to provide solid state telephone networks which eliminate inductive components and are accordingly readily amenable to hybridization.